In bicycle generators or dynamos, the roller envelope connected to the rotor and adapted to contact the bicycle wheel is urged towards the wheel with a contact pressure which is generated by a spring. This contact pressure must normally be sufficiently great to prevent the cylinder or the roller from slipping, even in the most adverse weather conditions. However, as a result, the contact pressure can be too great, particularly in dry weather, and unnecessary wear therefore occurs, and the user of the bicycle also may have to exert an unnecessary degree of force.